Yes, I agree with you and you are right its not without precedence. I think we had this same discussion about Oro and Edo Tensei as well too. Anyway you are right, there are any number of reasonable theories as to how it could happen but there is a difference between theorizing how things could have happened and Kishi telling us how they happened. Anyway this wouldn't be a problem if the Narutoverse just didn't have any rules but Kishi insists on giving his world rules that he just ignores whenever it suits him. That discussion about Naruto's shadow clones in the anime thread in a good example.
Anyway, Minato and Edo Tensei aren't what I'm interested in right now. I wanna know what you think about Itachi getting Susano'o after killing his best friend. Tsukiyomi and Amaterasu are one thing because for the most part they are simple enough and its reasonable to think that when he gained MS he got those abilities and he practiced and refined them into what they are, similar to Kakashi who I'm sure will perfect his time/space doujutsu in time, but Sasano'o.